A magnetism detecting apparatus incorporating a saturable type magnetic sensor can detect magnetism with high precision (Published Unexamined Japanese Patent Application No. 1-308982).
FIG. 5 is a block diagram showing a schematic arrangement of a magnetism detecting apparatus using a saturable type magnetic sensor. A rectangular wave generating circuit 1 outputs an AC rectangular signal e.sub.1 having a predetermined period T.sub.1, as shown in FIG. 6. A 0-volt line is located in the middle of the signal waveform of the rectangular signal e.sub.1. The rectangular signal e.sub.1 output from the rectangular wave generating circuit 1 is input to a differentiating circuit 2. The differentiating circuit 2 differentiates the rectangular signal e.sub.1. As a result, the differentiating circuit 2 outputs a pulse signal having a trigger waveform, which is synchronized with the leading and trailing edges of a rectangular wave included in the rectangular signal e.sub.1.
The pulse having the trigger waveform, output from the differentiating circuit 2, is applied, as an AC excitation signal e.sub.2, to a magnetic sensor 4 through an impedance element 3 constituted by a resistor. This magnetic sensor 4 is constituted by a rod-like ferromagnetic core 5 and a detection coil 6 wound around the core 5. The AC excitation signal e.sub.2 is applied to one end of the detection coil 6 of the magnetic sensor 4 through the impedance element 3. The other end of the detection coil 6 is grounded. The terminal voltage of the detection coil 6 is extracted as a detection signal e.sub.0 obtained by the magnetic sensor 4 and is input to a voltage detecting circuit 7.
The voltage detecting circuit 7 comprises a positive detector for detecting a positive peak voltage +Va of the detection signal e.sub.0 shown in FIG. 6, a negative detector for detecting a negative peak voltage -Vb of the detection signal e.sub.0, and an adder for adding the peak voltages +Va and -Vb detected by the respective detectors. An output voltage V.sub.0 corresponding to a magnetic field strength detected by the magnetism detecting apparatus is obtained by the voltage detecting circuit 7.
An operation principle of the magnetic sensor 4 will be described below with reference to FIGS. 7 to 11.
AC power having an AC voltage waveform shown in FIG. 7 is applied to the detection coil 6 of the magnetic sensor 4 through the resistor of the impedance element 3. In this case, a voltage e0 generated across the two ends of the detection coil 6 is determined in correspondence with a resistance R of the resistor and an impedance Zs of the detection coil 6. That is, EQU e.sub.0 =e.multidot.Zs/(R+Zs) (1)
where e is the applied voltage value.
Since the detection coil 6 is wound around the ferromagnetic core 5, the voltage e.sub.0 changes in proportion to the impedance Zs and the magnetic permeability of the core 5.
Assume that an AC current is supplied to the detection coil 6 while no external magnetic field is applied to the magnetic sensor 4. In this case, the magnetic permeability characteristic of the core 5 changes in accordance with the hysteresis characteristic of the core 6, as shown in FIG. 10. Note that reference symbol n denotes the number of turns of the coil; and i, a coil current.
For this reason, an output voltage generated across the two ends of the detection coil 6 has a waveform shown in FIG. 8. In the absence of an external magnetic field, positive and negative waveforms are symmetrical, and a positive voltage V.sub.1 is equal to a negative voltage V.sub.2.
If an external magnetic field is applied in this state, a magnetic flux crossing the core 5 becomes a composite magnetic flux constituted by the magnetic field generated by the detection coil 6 and the external magnetic field. As a result, the waveform of a voltage generated across the two ends of the detection coil 6 exhibits V.sub.1 &gt;V.sub.2, as shown in FIG. 9.
The external magnetic field, therefore, can be indirectly measured by obtaining the difference between the positive and negative voltages V.sub.1 and V.sub.2 of an output voltage generated across the two ends of the detection coil 6.
By using the magnetic sensor 4 of such a saturable type, output voltages V.sub.0 of 0 to 500 mV can be obtained with respect to small magnetic flux densities of 0 to 10 gauss, as shown in FIG. 11.
An operation of the magnetism detecting apparatus shown in FIG. 5 will be described below with reference to the above-described operation principle and the timing chart in FIG. 6.
The rectangular signal e.sub.1 output from the rectangular wave generating circuit 1 is converted into the pulse-like AC excitation signal e.sub.2 by the differentiating circuit 2. The signal e.sub.2 then flows in the detection coil 6 of the magnetic sensor 4 to excite the core 5. Note that the AC excitation signal e.sub.2 is set to a current value at which the core 5 is magnetized up to a saturation region. In this state, therefore, the waveform of the detection signal e.sub.0 indicated by the terminal voltage of the detection coil 6 exhibits a constant amplitude, as shown in FIG. 6.
In a state wherein an external magnetic field does not cross the saturated magnetic field generated by the detection coil 6, the positive and negative peak values Va and -Vb of the waveform are equal to each other, as indicated by a detection signal e.sub.0A on the left side of FIG. 6. If, however, an external magnetic field approaches the core 5 excited to the saturation region and crosses the saturated magnetic field formed by the detection coil 6, although no change in amplitude value occurs, positive and negative peak values Va and -Vb change, as indicated by a detection signal e.sub.0B on the right side in FIG. 6. These peak values Va and -Vb are detected by the above-mentioned positive and negative detectors to be converted into a DC voltage. The above-mentioned adder adds the peak voltages +Va and -Vb respectively detected by the positive and negative detectors to obtain a difference voltage (Va-Vb). The voltage detecting circuit 7 outputs this difference voltage (Va-Vb) as an output voltage V.sub.0. This output voltage V.sub.0 corresponds to the external magnetic field applied to the magnetic sensor 4. Therefore, a magnetic field strength can be detected by this magnetism detecting apparatus.
By using a saturable type magnetic sensor in this manner, the magnetic detection sensitivity of the apparatus can be improved, and the detection precision can be increased because a measurement result is substantially free from the influence of zero point variations due to variations in ambient temperature and the like, as compared with a magnetism detecting apparatus using a Hall element or a magnetoresistive element.
In this magnetism detecting apparatus, the ferromagnetic core 5 of the magnetic sensor 4 must be magnetized to the saturation region by supplying an AC excitation current to the detection coil 6 wound around the core 5. In order to accurately detect the positive and negative peak values Va and -Vb of the detection signal e.sub.0 output from the magnetic sensor 4, a pulse signal having a trigger waveform is used as the AC excitation signal e.sub.2 to be applied to the magnetic sensor 4 through the impedance element 3. Because of this trigger waveform, a high-frequency current flows in the detection coil 6. That is, in order to magnetize the core to the saturation region by using the pulse signal having the trigger waveform, an excitation current of, e.g., several 100 mA is required.
For this purpose, the voltage of the AC excitation signal e.sub.2 must be greatly increased. For example, even in the compact magnetic sensor 4, the above-mentioned voltage must be 15 to 25 V.sub.P--P. Since the rectangular wave generating circuit 1 needs to output the rectangular signal e.sub.1 having a peak value of 15 to 25 V.sub.P--P, a DC power source for applying a high voltage of 15 to 25 V is required in addition to a 5-V DC power source used in a general TTL circuit. As a result, the circuit arrangement is complicated, and the overall apparatus is increased in size. At the same time, the manufacturing cost is greatly increased.
In addition, since pulse-like noise due to pulses contained in the detection signal e.sub.0 may be mixed with the output voltage V.sub.0 from the voltage detecting circuit 7, a filter for removing such pulse-like noise is required.